The present invention relates to optical fiber sensor devices employing a fiber optic resonant ring. A fiber optic resonant ring, in conjunction with appropriate signal processing optoelectronics, responds to rotation with respect to a fixed inertial frame of reference, and can be employed as a highly sensitive and stable gyroscope in an inertial measurement system such as a vehicular navigation system. The sensitivity and utility of resonant fiber optic gyroscopes is limited by various environmental perturbations which can produce signal effects dominating the rotationally induced effects of interest. The architectures for resonant fiber optic gyroscopes (RFOGs) have evolved toward constructions yielding greater sensitivity, and the different architectures have progressively overcome each of the previously troublesome sources of environmental noise, while in each case being subject to a further environmental effect as its limiting constraint. The development of RFOGs has proceeded generally as follows.
The first RFOGs used resonant ring cavities fabricated out of single-mode (SM) optical fibers. The ring resonator consists of a closed loop of fiber configured as a coil and directionally coupled to an input/output fiber path. See FIG. 1. A fiber optic coupler, a four-port device with a given coupling ratio, is used to apply probe signals and to read out the transfer function (dips) of the cavity.
Single-mode fibers guide two nearly degenerate orthogonal polarization modes, which can easily couple to one another because of nearly matching propagation constants β. The degeneracy allows changing of the state of polarization, denoted SOP herein, of the light propagating in the fiber, so that multiple time-varying resonances occur as environmental conditions perturb the RFOG. Amplitude and separation will change for both SOPs. The signal processing generally consists of a servo of laser probe wavelength locked to cavity resonant dip; the detector cannot distinguish SOP and the electronics has problems locking to multiple dips. The drift in resonances and varying SOP is manifested as random drift in the sensed parameters constituting the gyro output, rendering the SM fiber construction useless for navigation-grade applications.
Subsequently, with the development of effective polarization-maintaining (PM) optical fibers, RFOGs were fabricated using this type of fiber and a resonant ring configuration essentially like that of the early SM fiber devices. In a PM fiber, a high degree of anisotropy, usually created by stress-induced fiber birefringence, breaks the degeneracy between the two orthogonal polarization modes. Because the modes in a PM fiber propagate at different β velocities, polarization cross-coupling, denoted PCC herein, is reduced to negligible levels, yielding a predictable SOP of light guided in the fiber. For example, if linearly polarized light is launched into the fiber such that the direction of polarization is aligned with one of the fiber's axes of symmetry, the SOP remains linear. There is a small cross-coupled component which arises when the light has traversed a great length of fiber, the magnitude of which is a function of fiber design and fabrication, winding, and input axis alignment; there are also PCC effects in the coupler and in other PM fiber components, e.g., splices used in construction of a complete gyroscope. When a resonator is fabricated out of PM fiber and the two modes propagate at different velocities, there are two resonances of differing orders. These two resonances are nominally independent for low PCC values in the ring assembly. When a pair of well-aligned polarizers are used on the input and outputs of the resonator, one resonance is greatly reduced with respect to the orthogonal dip. When this system is used in conjunction with the servo electronics to lock on a single dip it results in less output parameter drift when this type of fiber is used in an RFOG. Unfortunately, since the two resonances are of different order, environmental perturbations cause them to overlap occasionally, again resulting in unacceptable gyro drift and loss of output signal tracking. Even for extreme polarization isolation in the resonator components and assembly, when the resonances overlap, large coupling can occur due to phase matching conditions.
The next improvement to RFOG design included single-polarization (SP) optical fiber, again using essentially the same general resonant ring architecture as shown in FIG. 1. This design involves a modification of the PM fiber design, which, in addition to the aforementioned property of mode isolation, has the property that it preferentially attenuates one of the polarization modes. The quality of an SP fiber depends on the difference of mode attenuation for a given length of fiber, and is called the polarization extinction ratio (hereinafter PER). It is limited by the PCC of the fiber. When SP fiber is used in a resonant ring, the preferred mode resonates as before, and the attenuated mode does not resonate because it is extinguished at a rate that prevents a resonance from occurring in that mode. This approach results in a ring having only one resonant mode, yielding a truly single-channel resonant ring. Such RFOGs are capable of low drift rates and navigation-grade performance.
The foregoing is believed to represent the state-of-the-art technology for the construction of RFOG resonant rings.
The resonant ring used in the best RFOGs is presently made by a lapped coupler method, in which the pigtails of a wound fiber coil are each bonded into one of a pair of coupler block substrates, and surface region of each substrate is polished to almost reach the fiber core. When the two polished halves are assembled, evanescent coupling is effected between the entrance and exit pigtails so that the intermediate segment of the fiber forms a closed ring path making a resonant cavity of the fiber coil. During assembly the coupling ratio of the coupler so formed is optimized to equal the sum of cavity losses, and the two halves are then bonded in place to form a permanent assembly.
One problem with this approach is that the bond formed does not remain stable over environmental extremes, but causes the coupling ratio to shift, which reduces resonance amplitude, and the performance of the resonator to degrade. Another problem with the lapped coupler approach is that when this technique is used with a PM or especially with an SP fiber, the internal fiber stress which maintains fiber birefringence is perturbed by the partial removal of the fiber cladding and stress regions. SP fiber, which is the preferred fiber for RFOGs, is expensive and particularly difficult to fabricate into a reliable coupler using lapping, since the high stress value within the fiber is prone to degradation when the fiber is worked.
Catastrophic failure, polarization cross-coupling degradation, and extreme coupling ratio shift are often observed for this type of fiber even in a laboratory environment.
Because of these instabilities, applicant has determined that fused couplers are a preferred coupler technology for the fabrication of RFOGs. As demonstrated for SM fiber, fused components are well suited for mass production, have low losses and polarization cross-coupling, and most importantly, have been repeatably demonstrated to be environmentally rugged. However, these couplers, if made with PM or SP fiber, require the fiber stress regions be of the same refractive index as the surrounding. This property is incompatible with current SP fiber design. Special fibers with a matched stress region (MSR) design may be required to achieve low loss PM fused couplers.
Reverting to an RFOG design with a PM fiber rather than an SP fiber for the coil would allow the use of fused couplers, but would require the incorporation of a polarizer into the ring itself in order to obtain the instrument sensitivity of a single polarization resonant ring. But polarizer constructions made from PM fibers tend to have a lapped surface, and thus might be expected to re-introduce the environmental instabilities or manufacturing complexities of lapped coupler technology. A polarizer could be made by simply splicing in a length of SP fiber, by an arc fusion splice, but this would tend to introduce back reflections due to the mismatch of the fiber properties.